2 Burns & McDonnell

By Jon A. Schmidt, P.E., and Barie B. Brettmann, P.E., S.E.

Events over the last decade haveheightened awareness of the threat ofterrorist attacks using explosives. TheUnited States government has fundedresearch into methods for protectingbuildings against such attacks, andhas produced guidelines, particularlyfor high-risk projects such as militaryfacilities and embassies. Followingthe events of September 11, 2001, theprivate sector is increasingly consider-ing similar protection for its facilities,especially so-called "icon buildings"that are perceived to be attractive tar-gets for future terrorist actions. Thereare two key strategies that must beemployed in protective structuraldesign: 1) hardening to withstandblast loads and 2) preventing progres-sive collapse. This article, the first of atwo-part series, discusses hardeningtechniques.

Standoff DistancesThe best way to protect a buildingfrom blast loads is to ensure thatexplosives are kept as far away from itas possible. The effects of any blastcan be normalized by the scaled dis-tance parameter Z = R / W 1/3, whereR is the distance from the explosionand W is the equivalent TNT weightof the charge, determined from pub-lished tables when another explosivematerial is used for design. This rela-tionship is known as "cube-root scal-ing" and implies that by doubling thedistance from the building at whichan explosion must occur, the size ofthe explosive charge must beincreased by a factor of eight to havethe same destructive effect. The sig-nificance of this relationship for physi-cal security is that a larger explosive ismore difficult to transport and easierto detect.

Protective Structural Design:Resisting Blast Loads

Consequently, the first set of criteriathat must be established whendesigning a facility against terroristattack is the combination of standoffdistances that will be provided andexplosive charge sizes that must beaccommodated:

• The most severe explosive threat toa building is likely to be a station-ary or moving vehicle bomb.Ideally, to protect against such athreat, the building owner shouldestablish a secure perimeter as farfrom the building as possible, sothat any explosive large enough tobe detected during a search wouldhave to be detonated at that dis-tance. This is usually a given atmilitary installations, but is notalways feasible for private construc-tion. Vehicle barriers, such as bol-lards or special planters, are analternative means of ensuring thatlarge bombs are kept away from thebuilding when no secure perimeteris available.

• The next potential threat is a bombsmall enough to escape detectionduring a vehicle search when asecure perimeter exists, or in anycase capable of being carried andplaced by hand. A terrorist coulddetonate such an explosive in avehicle while it is in the building'sparking lot or garage or on an adja-cent roadway, or could set the bombin a location close to the buildingwhere it would not easily beobserved. For this reason, designersand owners should avoid puttinglarge items, such as trash containersand equipment, near the building insuch a way that they would obscurean explosive charge from the viewof the building's occupants.

Although it is notpractical to design

buildings to withstandany conceivable

terrorist attack, thereare strategies

available to improvethe performance

of structures should one occur.

3

• Finally, it is possible for a terrorist toutilize indirect fire weapons such asgrenades and military mortars.Although it is not practical to pro-tect an individual building againstsuch an attack, it is desirable to pro-vide a minimum separation betweenneighboring buildings to limit collat-eral damage.

Building designers must work withowners to determine the availablestandoff distances and appropriateexplosive charge sizes for each ofthese conditions, balancing the natureand likelihood of each threat with theadditional construction costs associat-ed with protecting a structure againstit. Once these parameters are estab-lished, the design team can proceed todetermine the blast loads on thebuilding and specify structural andarchitectural elements that can with-stand them.

Blast AnalysisThe load on a structure from a nearbyexplosion takes the form of an almostinstantaneous increase in pressure to

a maximum value, followed by a briefperiod during which the pressuredecays back to its ambient value.This effect can be approximated usinga triangular impulse load. Thedesigner can estimate the peak mag-nitude and duration of the impulsefor a given combination of standoffdistance and explosive charge weightusing the scaled distance value andcurves published in the literature,including the 1999 American Societyof Civil Engineers report StructuralDesign for Physical Security: State of thePractice. These curves were devel-oped by the military on the basis ofanalysis and tests. Computer pro-grams are also available to performthis calculation.

Each element to be designed can usu-ally be modeled as a dynamic systemwith a single degree of freedom —often corresponding to its mid-spandeflection — and mass, stiffness, anddamping derived from its actual phys-ical configuration and properties. Thedesigner can then determine theexpected response of the elementusing additional published curves or a

computer program capable of per-forming a nonlinear time historyanalysis of the simplified system. Therelevant parameters are as follows:

• Ratio of the blast impulse durationto the element's natural period ofvibration.

• Ratio of the element’s dynamic ulti-mate capacity to its maximumdynamic load.

• Ratio of the element’s maximumexpected deflection to its maximumelastic (impermanent) deflection(ductility ratio).

• Ratio of the element’s span length toits maximum expected deflection(deflection ratio).

The dynamic ultimate capacity of theelement is based on material strengthsthat are increased somewhat becauseof the short duration of the blast load-ing. The maximum dynamic load issimply the product of the maximumblast pressure and the element’s tribu-tary area. The ductility and deflectionratios are indicative of the amount ofdamage to an element that is expectedin a blast event, which is limited by

Table 1

Suggested Design Parameters for Blast-Resistant Structures

Level ofProtection

Maximum Ductility RatioLevel ofDamage

None

Light

Moderate

Severe

Nature of Damage

Superficial

Repairable

Concrete

1

3

5

10

Masonry

1

2

3

5

Minimum Deflection Ratio

High

Moderate

Low

Very Low

Concrete

240

120

60

15

Masonry

240

180

120

60

Unrepairable,no collapse

Unrepairable,collapse imminent

Steel

1

5

10

20

Steel

240

120

60

10

4 Burns & McDonnell

Nonstructural elements must also bedesigned with blast loads in mind.Consequently, exterior wall openingsshould be minimized. Frames mustbe capable of withstanding loadsgreater than or equal to the full staticcapacities of the doors and windowswithin them. When a high or moder-ate protection level is desired, doorsand windows should remain in theirframes, even if they break or are oth-erwise unusable afterwards. For alow level of protection, doors shouldrebound out of their frames if they failat all, and windows should break insuch a way as to pose a minimal frag-ment hazard.

Thermally tempered glass (TTG) isoften preferred over conventionalannealed glass when blast resistanceis desired because it is typically fourto five times stronger and fracturesinto small regular fragments, ratherthan large irregular shards.Polycarbonate glazing also providesgood blast resistance because it usual-ly fails by cracking in place, ratherthan by shattering. The behavior ofthese materials is familiar to anyonewho has had an automobile side orrear window (TTG) or front wind-shield (polycarbonate) damaged in anaccident. Laminated combinations ofTTG and/or polycarbonate layers,with or without an exterior sacrificiallayer of annealed glass, can also bequite effective in withstanding explo-sions. However, these and other alter-natives can add considerable expenseto a building project.

ConclusionAlthough it is not practical to designbuildings to withstand any conceiv-able terrorist attack, there are strate-gies available to improve the perform-ance of structures should one occur.

Barie Brettmann is an associatestructural engineer, with almost 20years of experience specializing inthe design of public and privatebuildings. He joined Burns &McDonnell in 1997.

Jon Schmidt is a senior struc-tural engineer, specializing in theanalysis and design of aviationand industrial facilities. He hasworked for Burns & McDonnellsince 1994.

the level of protection that the struc-ture is required to provide to its occu-pants.

Table 1 summarizes four somewhatarbitrary levels of protection and sug-gests corresponding ductility anddeflection ratios for the three structur-al materials most commonly used inblast-resistant buildings. These limitsare loosely based on the recommenda-tions published in the United StatesArmy TM 5-1300 / Navy NAVFAC P-397 / Air Force AFR 88-22, Structuresto Resist the Effects of AccidentalExplosions. This publication actuallyspecifies maximum end rotations ofelements (Ø), which can be convertedto deflection ratios (D) by assumingthat plastic hinges form at fixed endsand at mid-span and then usingtrigonometry (D = 2 / tan Ø). Thedesigner should evaluate and adjustthe allowable ductility and deflectionratios carefully for the specific ele-ment, material, and failure modeunder consideration.

Reinforced concrete, properlydetailed, is the preferred material forblast-resistant structures because itusually is capable of safely sustainingsubstantial localized damage, has afair amount of redundancy, and canbe used for the walls themselves.Masonry can also be employed for thelatter, but must always be reinforcedand even then has a considerablyhigher potential for unacceptable brit-tle failure. Cavity walls are preferableto single wythes because the outerlayer of brick will absorb not only aportion of the blast pressure, but alsomany of the fragments produced bythe explosion. Structural steel, espe-cially when utilized in moment-resist-ing frames, can tolerate a considerableamount of deflection during a blastevent without collapse.

By maximizing standoff distancesand hardening key elements, design-ers can provide building occupantswith a reasonable chance of escapingdeath and serious injury in the eventof a nearby explosion.

Look for “Protective Structural Design:Preventing Progressive Collapse” in thenext issue of TechBriefs.